Intracavity frequency conversion of laser radiation

ABSTRACT

A laser device with frequency conversion, the device comprising a complex optical cavity comprising two cavity parts with two different levels of circulating intracavity power wherein there is placed at least one non-linear crystal ( 30 ) is placed within the cavity part of higher circulating power and an active medium ( 21 ) in the cavity part of lower circulating power, the power enhancement achieved in two steps and the total enhancement being the product of the enhancement factors in each step, providing additional freedom in design allowing both the condition for high enhancement of the interacting laser power inside the intracavity non-linear crystal and the condition for maximum power output from the laser to be satisfied simultaneously and wherein said complex optical cavity the first cavity part provides the initial step of power enhancement and comprises at least a laser cavity back mirror ( 20 ), highly reflective about a laser radiation fundamental frequency ω, and an active (gain) medium.

[0001] The present invention relates to frequency conversion of laserradiation by means of non-linear interaction of laser radiation with asuitable non-linear optical material. In particular the inventionrelates to intracavity Second Harmonic Generation (SHG), also calledFrequency Doubling (FD), and intracavity Optical Pararnetric Oscillation(OPO). The invention can be also applied to intracavity Third and FourthHarmonic Generation as well as for intracavity Raman Frequency Shifting(RFS).

[0002] In a non-linear frequency conversion process the efficiency forconversion of laser power at a fundamental frequency into power atcombined frequencies (for example, the second harmonic frequency) isstrongly dependent on the intensity of radiation interacting with thenon-linear optical material (non-linear crystals). In practice,interacting intensities within a range of 10⁶-10⁸ W/cm² are needed forachieving practically significant conversion efficiencies, even for thebest non-linear crystals known. To increase the interacting intensity itis common to focus the radiation incident upon the non-linear crystal.

[0003] In the case of continuous wave (CW) lasers of low and moderatepowers an increase of the intensity by focusing is still not enough forobtaining reasonable conversion efficiencies, and a further enhancementof laser light intensity inside the non-linear crystal remainsnecessary. This can be achieved by placing the non-linear crystal withinthe laser cavity, where radiation power is increased compared with thepower available from outside the cavity of the same laser, providingpower enhancement inside the crystal by tens or even hundreds of times.Such schemes are discussed in, for example, W. Koechner, “Solid-StateLaser Engineering”, Third Edition, Springer-Verlag, 1992.

[0004] However, such an intracavity arrangement (for example, for SHG orOPO), which provides a satisfactory condition for interaction within thenon-linear crystal, generally does not allow for high efficiency of alaser system as whole. This is because the condition for largeenhancement of the fundamental frequency power inside the laser cavityis at variance with the condition for maximum extraction of the poweravailable from the laser at a particular level of pump power supplied tothe laser.

[0005] For maximum out-coupling of the generated power from the laserthe useful cavity loss needs to be of a certain (optimal) value thatincreases with increase of pump power. The cavity loss comprises twoparts, the first part being the useful loss due to the laser output andthe second part being an internal (also called useless) cavity loss asthe result of inevitable power dissipation from the laser cavity due toscattering, re-absorption, residual reflection and/or transmission bycavity components, and other factors. For effective interaction insidethe non-linear crystal the power enhancement factor always needs to behigh. This can be achieved by decreasing the cavity loss to a value assmall as reasonably possible, which, however, results in the conditionfor out-coupling the laser power straying from its optimum value.

[0006] As a result of the relationship between the intracavity powerenhancement and optimal power extraction from the laser cavity, theoverall efficiency of intracavity SHG (or other intracavity frequencyconversion processes such as OPO) with respect to pump power applied tothe laser is low, usually far below 10%.

[0007] Another disadvantage of the above intracavity arrangement forfrequency conversion of laser radiation is high sensitivity of the laseroutput to small environmental changes, thermal effects, scattering byair, and the like. As the cavity loss is kept at a small value, smallexternal disturbances can noticeably change the balance between theuseful and internal loss of the laser cavity, leading to a strongvariation of the laser output. This decreases the laser stability andnecessitates stabilisation measures and tight tolerances in the lasercomponents used.

[0008] It is an aim of the present invention to eliminate or reduce oneor more of the above disadvantages of the intracavity frequencyconversion of laser radiation, in particular (but not exclusively) forintracavity SHG and intracavity OPO, providing improvement of overalllaser system efficiency and stability.

[0009] The inventor has established a new concept for intracavityfrequency conversion, the concept termed Double ENhanced IntraCAvityFrequency Conversion (DENICAFC) and, in particular but not exclusively,Double ENhanced IntraCAvity Frequency Doubling (DENICAFD) that is basedon using a complex cavity capable of enhancing power interacting withthe non-linear crystal in two steps (double enhancement) with respect tothe power available from outside the laser cavity.

[0010] According to a first aspect of the invention there is provided alaser device with frequency conversion, the device comprising a complexoptical cavity comprising two cavity parts with first and seconddifferent levels of circulating intracavity power wherein at least onenon-linear crystal is placed within the cavity part of highercirculating power and an active medium in the cavity part of lowercirculating power.

[0011] The advantage of achieving power enhancement in two steps, withthe total enhancement factor being the product of the enhancementfactors in each step, is that it provides additional freedom in designallowing both the condition for high enhancement of the interactinglaser power inside the intracavity non-linear crystal and the conditionfor maximum power output from the laser to be satisfied simultaneously.These two steps of enhancement will now be explained in more detail.

[0012] In said complex optical cavity the first cavity part provides theinitial step of power enhancement and comprises at least a laser cavityback mirror, highly reflective about a laser radiation fundamentalfrequency ω, and an active (gain) medium.

[0013] The first cavity part may also include polarisation and/ orwavelength selectors. The first cavity part may also include cavity lossmodulators, for example as used for Q-switching. There will generally beprovided suitable pumping means for the active (gain) medium.

[0014] It will be understood that the term “active (gain) medium” refersto any suitable laser material, in particular but not necessarily asolid state (for example crystalline, glassy, semiconductor,semiconductor compound such as Vertical Cavity Surface EmittingLaser—VCSEL—structures, etc.) laser material that, being pumped orexcited appropriately, is capable of amplifying and emitting radiationwithin a certain spectral range.

[0015] The second cavity part of the complex cavity comprises a resonantreflector incorporating at least one non-linear crystal. As a result ofhaving an optical non-linearity built-in this cavity part functions as anon-linear resonant reflector at the laser fundamental frequency, ω. Thebackward reflectivity of the non-linear resonant reflector, with respectto radiation incident upon it from the first cavity part, isself-regulated by the presence of a non-linear crystal to be as close tothe optimal value for out-coupling the fundamental frequency powercirculating within the first cavity part.

[0016] Placing a non-linear optical medium for frequency conversionwithin a resonant reflector layout makes use of the power enhancingproperty of the resonant cavity part. This also gives rise to and use ofan additional feature of the resonant reflector, namely, self regulationof its backward reflectivity at the fundamental frequency ω close to theoptimal value with regard to power out-coupling from the first part ofthe laser cavity.

[0017] Thus the second part of the laser cavity as described above,being the non-linear resonant reflector, provides the second step ofpower enhancement for intracavity frequency conversion, and at the sametime performs as an optimal output coupler, therefore allowing formaximum extraction of power. The self regulation property results inimproved stability and more relaxed tolerances in manufacture and/oralignment of cavity components.

[0018] In one preferred embodiment, suitable for intracavity frequencyconversion, said second cavity part of said complex optical cavity isformed by two end mirrors, highly reflective about the fundamental laserradiation frequency ω, and a beamsplitter mirror, partiallytransmitting/reflecting about the fundamental laser radiation frequencyω, wherein all three mirrors are arranged in a configuration to provideresonant reflection backward to the first cavity part, and incorporatesa non-linear element within the optical path between the beamsplitterand one of said end mirrors.

[0019] The frequency conversion may include processes such as second,third and fourth harmonic generation, optical parametric oscillation andintracavity Raman frequency shifting.

[0020] In addition to the above reflectivity conditions about the laserradiation fundamental frequency ω for the mirrors of the second cavitypart comprising the non-linear resonant reflector part of the complexoptical cavity, the reflectivity of these mirrors about a combinedfrequency (for example the second harmonic, or OPO generated, or Ramanshifted frequency) can be chosen so as to output the laser radiationpower at said combined frequency in desired direction(s).

[0021] For the case of uni-directional output, one of the end mirrors ofthe non-linear resonant reflector part is made highly reflective at thecombined frequency, while at least one of the beamsplitter and thesecond end mirror at the combined frequency is made relativelytransmissive, dependent on the desired direction of the output power.

[0022] In another preferred embodiment, suitable for intracavityfrequency tripling and quadrupling, said second cavity part of the lasercavity as described above incorporates two non-linear crystals, onephase-matched for SHG (ω+ω) and another phase-matched matched fortripling (ω+2ω) or quadrupling (2ω+2ω), wherein in addition to thereflectivity conditions about the laser radiation fundamental frequencyω all three mirrors of the second cavity part of the laser cavity alsocan be highly reflective about the frequency 2ω to enhance the secondharmonic power within the non-linear crystals as well. The choice forreflectivity of these mirrors at the third or fourth harmonic remainsdependent on the desired direction of the output.

[0023] The back mirror of said first cavity part may be fabricated onthe appropriate end of the active (gain) medium.

[0024] The complex cavity is preferably configured so as to maximiselaser output, and hence to maximise the laser efficiency with respect tothe pump power supplied to the gain medium. The complex cavity ispreferably configured so as to provide minimal sensitivity of outputpower to cavity loss variations caused by external disturbances.

[0025] The mirror curvatures of first and second parts of the complexlaser cavity can be chosen and the mirrors configured so as to match thetransverse and longitudinal mode structure of the laser beam within thecomplex cavity.

[0026] There is further provided a method of laser radiation frequencyconversion in accordance with the apparatus as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Embodiments of the invention will now be described, by way ofexample only, with reference to the accompanying drawings, in which:

[0028]FIG. 1 shows a known laser cavity layout commonly used forintracavity SHG;

[0029]FIG. 2 shows a laser cavity configuration suitable for doubleenhanced intracavity frequency doubling according to one embodiment ofthe invention;

[0030]FIG. 3 shows an alternative cavity configuration suitable fordouble enhanced intracavity frequency doubling;

[0031]FIG. 4 shows a laser cavity configuration suitable for doubleenhanced intracavity frequency tripling or quadrupling.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0032]FIG. 1 shows a layout of laser cavity principal optical componentspreviously proposed for intracavity frequency doubling, where the cavityback mirror 20, active (gain) medium 21, non-linear crystal 30, andmirror 31 comprise a linear type (as opposed to a ring geometry) lasercavity, folded by cavity folding mirror 32 for allowing uni-directionalextraction of generated power at frequency 2ω (second harmonic). Thewaved arrow 34 indicates that an appropriate pump of the active medium21 is arranged. In such a layout all three mirrors 20, 32, and 31 aremade highly reflective at the laser fundamental frequency. Commonly, thereflectivity of these mirrors are made as close to 100% astechnologically possible by mirror manufacturers. For uni-directionalextraction of second harmonic power the mirror 31 is also made highlyreflective at frequency 2ω, while the folding mirror 32 is made astransparent as possible at frequency 2ω. Thus the cavity resonates andenhances the circulating laser power at the fundamental frequency ω.Power at a combined frequency of 2ω, being the combined frequencygenerated by non-linear interaction within the crystal 30, is ejected asindicated by path 36 via the folding mirror 32, after each“forward-backward” (round-trip) passing through non-linear crystal 30.

[0033] In this scheme the only useful loss of the generated fundamentalfrequency power is by non-linear conversion into the second harmonicpower by the non-linear crystal 30, and is usually less than 1% percavity round-trip in the case of continuous wave (CW) lasers of small ormoderate powers (in the range of milliwatts to a few watts). Despitecareful measures to minimise the internal cavity loss by usinganti-reflection (AR) coatings at the non-linear crystal and activemedium end surfaces and forming the cavity mirror 20 onto the rearsurface 38 of the laser gain medium 21, the remaining reflections ofAR-coated surfaces and residual transmission of the cavity mirrors atthe fundamental frequency ω, along with cavity diffraction loss,scattering and re-absorption inside the active medium and non-linearcrystal, introduce a significant useless loss of the generatedfundamental power that can be comparable and even in excess of 1% percavity round trip. As a result, the total cavity loss is dominated bythe internal (useless) part therefore making overall laser efficiencyrather small with respect to the pump power and very sensitive to anyoutside disturbances.

[0034]FIG. 2 shows an embodiment of an apparatus for implementing amethod of what we shall term double enhanced intracavity frequencydoubling (DENICAFD). The first part of a complex cavity comprises asbefore the cavity back mirror 20, with high reflectivity at the laserfundamental frequency ω, and active (gain) medium 21. It may alsoinclude other optical elements such as polarisation and/or wavelengthselectors 40, 42, and cavity loss modulator 44. The cavity back mirror20 can also be deposited onto the rear surface 38 of the active medium21. The waved arrow 34 in the drawing indicates that an appropriate pumpof the active medium 21 is arranged.

[0035] The second part of the laser cavity makes up the non-linearresonant reflector and comprises two end mirrors 45 and 46, highlyreflective at the laser fundamental frequency ω, a beamsplitter mirror48 being partially reflective at the frequency ω, and a non-linearcrystal 30 of an appropriate orientation to provide the phase matchingcondition for frequency doubling. To achieve the highest efficiency ofthe laser, the reflectivity of the mirrors 20, 45 and 46 should be madeas close to 100% as technologically possible at the required fundamentallaser frequency, ω. The appropriate partial reflectivity value of thebeamsplitter mirror 48 is any value lying within some range around thereflectivity that would be chosen for an optimal output coupler, if sucha coupler were to be used (instead of the non-linear resonant reflector)simply to extract maximum power from the laser at the fundamentalfrequency. The skilled person is familiar with the criteria forestablishing the optimal reflectivity for such an arrangement.

[0036] In the cavity layout of FIG. 2 the non-linear crystal 30 isshown, by way of example only, within the non-linear resonant reflectorpath between the mirrors 48 and 45, that is, angled to the optical axisof the first part of the cavity. The non-linear crystal can be alsoplaced in the path between the mirrors 46 and 48. There is no specificrestriction on choosing the angle (for example 90°) of folding thenon-linear resonant reflector part of the laser cavity with respect tothe optical axis of the first cavity part, (and accordingly, the angleof tilting the beamsplitter mirror 48) except those dictated byconvenience of design and alignment.

[0037] With the above reflectivities of the laser cavity mirrors thefundamental frequency power circulating inside the cavity of FIG. 2 hastwo different levels: a lower level within the cavity path between thecavity back mirror 20 and beamsplitter mirror 48, and a higher levelwithin the non-linear resonant reflector path between the mirrors 46, 48and 45. The lower level, however, is already an enhanced level of thefundamental frequency power as compared with what it would be outsidethe laser cavity. Thus, for the non-linear crystal being placed withinthe non-linear resonant reflector part of the laser cavity there are twostages of enhancement of the fundamental frequency power. Due to theoptical non-linearity being incorporated within the resonant reflector,the backward reflectivity (in the direction of the cavity back mirror20) is self regulated to be close to the optimal value for out-couplingthe fundamental frequency power that is circulating within first part ofthe laser cavity. This provides the condition for the maximum secondharmonic output with respect to the pump power supplied to the active(gain) medium and hence the optimum laser efficiency, and providesminimal sensitivity of the laser output to the laser cavity internalloss variations due to external disturbances and limited spec tolerancesof the laser cavity components.

[0038] To arrange for the unidirectional output of the second harmonicpower from the laser, the reflectivitities of the mirrors 45, 48 and 46at the frequency 2ω must be chosen appropriately. In the case as shownin FIG. 2, for example, the mirror 45 is also highly reflective at 2ωand the beamsplitter mirror 48 is highly transmittive at 2ω. Hence, thesecond harmonic output power is directed as shown by path 36.Alternatively, for the second harmonic power to be output through themirror 46, the latter should be highly transmittive at the frequency 2ω,while both the mirror 45 and the beamsplitter mirror 48 should be highlyreflective at 2ω.

[0039]FIG. 3 shows an alternative laser cavity layout for theimplementation of double enhanced intracavity frequency doubling. Therequirement for the cavity mirror's reflectivities at the laserfundamental frequency are the same as in the case of the layout of FIG.2, except for the reflectivity value of the beamsplitter mirror 48. Thereflectivity of mirror 48, for optimal performance of the laser at thefundamental frequency ω, must in this case be approximately equal to thetransmission of the beamsplitter mirror 48 of the layout of FIG. 2.There are no specific restrictions either with regard to choosing theangle between optical axis of first and second parts of the cavity, orwith regard to in which path of the second part of the cavity (resonantreflector) to place a non-linear crystal. Again, as in the case of thelayout shown in FIG. 2, the reflectivities of the mirrors 45, 48 and 46at the second harmonic frequency 2ω are chosen appropriately to provideuni-directional second harmonic output in the desired direction.

[0040] In both the above cases the curvatures of the mirrors comprisingthe complex laser cavity and distances between them are chosen such asto match the transverse and longitudinal mode structure of the laserbeam within the cavity. The criteria for this selection are familiar tothose skilled in the art.

[0041]FIG. 4 shows an extension of the cavity layout of FIG. 2 for usein double enhanced intracavity frequency tripling. In this case a secondnon-linear crystal 50, in an orientation to phase-match the sumfrequency process (ω+2ω), is placed within the resonant reflector partof the laser cavity. The mirrors 45, 46, and made 48 are highlyreflective about the second harmonic frequency 2ω, while the conditionfor their reflectivity about the fundamental frequency ω remains thesame as in the case of FIG. 2. Accordingly, for uni-directional outputof the third harmonic power indicated by path 52, the mirror 45 is alsohighly reflective at the frequency 3ω while the beamsplitter mirror 48is highly transmitting at the frequency 3ω.

[0042] By choosing the phase matching conditions of the secondnon-linear crystal 50 in the layout of FIG. 4 for further doubling thesecond harmonic frequency 2ω, with appropriate reflectivities of theresonant reflector mirrors at the fourth harmonic frequency 4ω, a doubleenhanced intracavity frequency quadrupling can be achieved.

[0043] The skilled reader will appreciate that the invention is notlimited to the specific implementations and applications detailed above.The configurations discussed above are also suitable for implementationof this invention in more general sense of what may be termed doubleenhanced intracavity frequency conversion (DENICAFC), for exampleintracavity optical parametric oscillation (OPO) or intracavity Ramanfrequency shifting (RFS). In such cases the phase matching conditionsfor a non-linear crystal as well as reflectivity of the resonantreflector mirrors have to be chosen accordingly about frequencies of theidler and signal waves (OPO) or about corresponding Stock's frequencies(RFS).

What is claimed is:
 1. A laser device with frequency conversion, thedevice comprising a complex optical cavity comprising two cavity partswith first and second different levels of circulating fundamentalfrequency intracavity power wherein at least one non-linear crystal isplaced within the cavity part of higher circulating power and an activemedium in the cavity part of lower circulating power.
 2. The laserdevice as claimed in claim 1 wherein a first cavity part provides theinitial step of power enhancement and comprises at least a laser cavityback mirror, highly reflective about a laser radiation fundamentalfrequency ω, and an active (gain) medium.
 3. The laser device as claimedin claim 1 wherein the first cavity part also includes polarizationand/or wavelength selectors.
 4. The laser as claimed in claim 1 whereinthe first cavity part also includes cavity loss modulators, forQ-switching.
 5. The laser device as claimed in claim 2 wherein there isprovided suitable pumping means for the active (gain) medium.
 6. Thelaser device as claimed in claim 2 wherein the active (gain) medium isany suitable laser material.
 7. The laser device as claimed in claim 6where the laser material is a solid state laser material that, beingpumped or excited appropriately, is capable of amplifying and emittingradiation within a certain spectral range.
 8. The laser device asclaimed in claim 2 wherein a second cavity part of the complex cavitycomprises a resonant reflector incorporating at least one non-linearcrystal.
 9. The laser device as claimed in claim 8 wherein the secondcavity part functions as a non-linear resonant reflector at the laserfundamental frequency, ω.
 10. The laser device as claimed in claim 1wherein the backward reflectivity of the non-linear resonant reflector,with respect to radiation incident upon it from the first cavity part,is self-regulated by the presence of a non-linear crystal to be as closeto the optimal value for out-coupling the fundamental frequency powercirculating within the first cavity part.
 11. The laser device asclaimed in claim 8 wherein the second part of the laser cavity, beingthe non-linear resonant reflector, provides the second step of powerenhancement for intracavity frequency conversion, and at the same timeperforms as an optimal output coupler, therefore allowing for maximumextraction of power.
 12. The laser device as claimed in claim 8 whereinsaid second cavity part of said complex optical cavity is formed by twoend mirrors, highly reflective about the fundamental laser radiationfrequency ω, and a beamsplitter mirror, partiallytransmitting/reflecting about the fundamental laser radiation frequencyω, wherein all three mirrors are arranged in a configuration to provideresonant reflection backward to the first cavity part, and incorporatesa non-linear element within the optical path between the beamsplitterand one of aid end mirrors.
 13. The laser device as claimed in claim 1wherein frequency conversion includes processes such as second, thirdand fourth harmonic generation, optical parametric oscillation andintracavity Raman frequency shifting.
 14. The laser device as claimed inclaim 2 wherein, in addition to the reflectivity conditions about thelaser radiation fundamental frequency ω for the mirrors of the secondcavity part comprising the non-linear resonant reflector part of thecomplex optical cavity, the reflectivity of these mirrors about acombined frequency are chosen so as to output the laser radiation powerat said combined frequency in desired direction(s).
 15. The laser deviceas claimed in claim 14 wherein said combined frequency is the secondharmonic, or OPO generated, or Raman shifted frequency.
 16. The laserdevice as claimed in claim 1 wherein for the case of uni-directionaloutput, one of the end mirrors of the non-linear resonant reflector partis made highly reflective at the combined frequency, while at least oneof the beamsplitter and the second end mirror at the combined frequencyis made relatively transmissive, dependent on the desired direction ofthe output power.
 17. The laser device as claimed in claim 8 whereinsuitable for intracavity frequency tripling and quadrupling, whereinsaid second cavity part of the laser cavity as described aboveincorporates two non-linear crystals, one phase-matched for SHG (ω+ω)and another phase-matched for tripling (2+2ω) or quadrupling (2ω+2ω),wherein in addition to the reflectivity conditions about the laserradiation fundamental frequency ω all three mirrors of the second cavitypart of the laser cavity also can be highly reflective about thefrequency 2ω to enhance the second harmonic power within the non-linearcrystals as well.
 18. The laser device as claimed in claim 17 whereinthe choice for reflectivity of the mirrors at the third or fourthharmonic remains dependent on the desired direction of the output. 19.The laser device as claimed in claim 2 wherein the back mirror of saidfirst cavity part is fabricated on the appropriate end of the active(gain) medium.
 20. The laser device as claimed in claim 1 wherein thecomplex cavity is preferably configured so as to maximize laser output,and hence to maximize the laser efficiency with respect to the pumppower supplied to the gain medium.
 21. The laser device as claimed inclaim 1 wherein the complex cavity is preferably configured so as toprovide minimal sensitivity of output power to cavity loss variationscaused by external disturbances.
 22. The laser device as claimed inclaim 1 wherein the mirror curvatures of first and second parts of thecomplex laser cavity are chosen and the mirrors configured so as tomatch the transverse and longitudinal mode structure of the laser beamwithin the complex cavity.
 23. The laser device as claimed in claim 1wherein there is further provided a method of laser radiation frequencyconversion in accordance with the apparatus as described above.